Revision as of 13:06, 28 August 2009

Contributed by Ramon Crehuet

Problem

You have a PDB with disordered atoms, i.e. different atomic positions with occupancies that add up to 100%. From this PDB you want to create a new one having only one set of the disordered atoms. This can be necessary if you want to perform RMSD calculations or Molecular Dynamics simulations.

Solution

Bio.PDB is proficient in dealing with disordered atoms. Each disordered atom has a property indicating its alternative positions: atom.altloc. Usually there are only two alternative positions labelled 'A' and 'B'. The key is to save a PDB with the optional "select" argument. This argument needs to return a True value for the atoms that have to be saved. In the following example we save all not-disordered atoms and the 'A' positions of the disordered ones.

Discussion

It is trivial to change that to save 'B' altloc positions. One can even do more complicated selections based on other atom properties. The key is to generate a class that returns True or False for a given atom.
One could also think of deleting atoms with 'B' values in atom.altloc.

for atom in all_atoms: # all_atoms is a list containg all atomsif atom.altloc=='B': del atom

but that does not work, because it only deletes the local variable and not the PDB structure.